Conventionally, an automotive vehicle door is connected to a vehicle main body so as to be openable and closable. That is, provided between the vehicle main body and the vehicle door is a vehicle door hinge connecting the same to each other. Thus, the vehicle door is rotatably connected to the vehicle main body so as to be openable and closable. This vehicle door hinge generally has a female bracket securely provided to the vehicle main body, a male bracket securely provided to the vehicle door, and a rotation shaft member that is configured to rotatably connect the male bracket to the female bracket so as to be relatively rotated to each other. Further, provided between the rotation shaft member and the male bracket is a slide bush. This slide bush is configured to rotate integrally with the male bracket, so as to smoothly rotate the male bracket with respect to the rotation shaft member that is integrally connected to the female bracket.
In a coating line of a manufacturing process, the vehicle main body is applied with electrodeposition coating as a rust prevention measure in a condition in which the vehicle door is connected to the vehicle main body via the above-described vehicle door hinge. An example of this electrodeposition (ED) coating is a cation coating. In particular, the vehicle main body with the vehicle door connected thereto is dipped in a coating liquid in which water-based paint is dissolved. Thereafter, electrodes are respectively connected to the vehicle main body thus dipped and the coating liquid, and voltage is applied between these electrodes. Thus, an electric current flows through the vehicle main body and the vehicle door connected thereto via the vehicle door hinge, so that the coating liquid thereon can be electrodeposited thereon. As a result, an ED coating film having rust prevention performance can be formed on a surface of the vehicle main body including the vehicle door (for example, Japanese Laid-Open Patent Publication No. 60-110898).
Further, the electric current flows from the electrode connected to the vehicle main body to the vehicle door via the vehicle door hinge. The electric current flowing through this vehicle door hinge flows in the order of the female bracket, the rotation shaft member, the slide bush and the male bracket. These members are formed of electroconductive materials and are positioned in contact with each other. Therefore, the electric current can flow from the vehicle main body to the vehicle door via the vehicle door hinge.
On the other hand, in the above-described ED coating, the electric current can flow through the vehicle door hinge, so that the ED coating film can be formed on the vehicle door hinge. This ED coating film is also formed over a boundary portion between the rotation shaft member and the slide bush. Thus, when the slide bush is rotated relative to the rotation shaft member, the ED coating film formed over the boundary portion between the rotation shaft member and the slide bush can be cracked and peeled off. The ED coating film thus cracked and peeled off can be scattered at the time of subsequent spray coating, and can be included in a spray coating film. If the cracked or peeled coating film remains during spray coating film, a roughened surface called a so-called “coating irregularity” is generated on a surface of the spray coating film. This may lead to deterioration of product quality.
As a countermeasure against the generation of such an “uneven coating,” a method is conventionally known. In the method, an ED coating film is previously formed on only the rotation shaft member of the vehicle door hinge before the ED coating is applied to the entire vehicle main body. When the ED coating film is previously formed on only the rotation shaft member, electrical conductivity with respect to external contact can be reduced by the ED coating film. Thus, when the ED coating is performed on the entire vehicle main body after the ED coating film is formed, the coating liquid can be prevented from being electrodeposited by the previously formed ED coating film. As a result, the ED coating film can be prevented from being newly formed over the boundary portion between the rotation shaft member and the slide bush.
However, the previously formed ED coating film may lead to a reduction in conductive property due to the external contact. As a result, electricity supply performance for the female bracket and the slide bush contacting this rotation shaft member can be impaired. Thus, a stable electric current equivalent to the current fed to the vehicle main body cannot be fed to the vehicle door. As a result, the ED coating film can be non-uniformly formed on the vehicle door. This may lead to deterioration of the product quality.
Thus, there is a need in the art to provide an improved vehicle door hinge.